The last 25 years have shown significant advancements in the performance properties of high temperature superconductors. With those improving performance properties has come the desire to utilize HTS for the practical application of loss-free electrical power transmission and other energy saving applications in the development of the smart grid and other energy related applications.
Early efforts, sometimes termed the first generation (1G) HTS, to form wires did not yield technically or commercially acceptable solutions. The superconductors typically used wire drawing technology, often using variations on Bi2Si2Ca2O7. Unfortunately, the HTS was too brittle and costly due to high silver content for the intended applications.
Efforts in the so-called second generation (2G) HTS have focused on using a thin layer of high quality HTS usually (RE)BaCuO (also known as (RE)BCO, and where (RE) is a rare earth such as yttrium, samarium, gadolinium, neodymium, dysprosium, etc.) usually on a flexible metal substrate. Such 2G HTS is capable of maintaining a dissipation free state while still carrying large electrical currents even in the presence of a large magnetic field such as is found in motor and generator applications. Most proposed solutions adopt the addition of materials beyond the basic components of the HTS in order to provide extrinsic pinning centers. For example, prior art focuses on adding extra pinning centers by incorporating extra elements during film growth, e.g. Zr, Ta, Nb, Sn. Adding these extrinsic elements to (RE)BCO makes the process more complicated and also raises cost. Despite these clear drawbacks, the field relatively uniformly follows this approach. See, e.g., Foltyn et al., “Materials Science Challenges for High-Temperature Superconducting Wire”, Nature Materials, Vol. 6, September 2007, pp. 631-641, Maiorov, et al., “Synergetic Combination of Different Types of Defect to Optimize Pinning Landscape Using BaZrO3-doped YBa2Cu3O7”, Nature Materials, Vol. 8, May 2009, pp. 398-404, Zhou, et al., “Thickness Dependence of Critical Current Density in YBa2Cu3O7-δ Films with BaZrO3 and Y2O3 Addition, “Supercond. Sci. Technol. 22 (2009), pp. 1-5, Sung Hun Wee, Amit Goyal, and Yuri L. Zuev, “Growth of thick BaZrO3-doped YBa2Cu3O7-δ films with high critical currents in high applied magnetic fields”, IEEE Transactions on Applied Superconductivity, Volume 19, Number 3, June 2009, pp. 3266-3269, Amit Goyal, M. Parana Paranthaman, and U. Schoop, “The RABiTS Approach: Using Rolling-Assisted Biaxially Textured Substrates for high-performance YBCO superconductors”, MRS Bulletin, August 2004, pp. 552-561, T Aytug, M Paranthaman, E D Specht, Y Zhang, K Kim, Y L Zuev, C Cantoni, A Goyal, D K Christen, V A Maroni, Y Chen, and V Selvamanickam, “Enhanced flux pinning in MOCVD-YBCO films through Zr additions: systematic feasibility studies”, Superconductor Science and Technology Volume 23 (2010), Sung Hun Wee, Amit Goyal, Eliot D, Specht, Claudia Cantoni, Yuri L. Zuev, V, Selvamanickam, and Sy Cook, “Enhanced flux pinning and critical current density via incorporation of self-assembled rare-earth barium tantalate nanocolumns within YBa2Cu3O7-δ films”, Rapid Communications, Physical Review B Volume 81 (2010), Amit Goyal, Claudia Cantoni, Eliot Specht, Song-Hun Wee, “Critical current density enhancement via incorporation of nanoscale Ba2(Y,Re)TaO6 in REBCO films”, US Patent Application publication US2011/0034338A1, Feb. 10, 2011.
Another approach which has proved less promising to date is introducing microstructural defects, into the HTS. See, for example, Selvamanickam published US Application 2011/0028328 which suggests use of a complex surface treatment including the formation of nanorods grown on an array of nanodots, especially nanorods of BZO. In a document dated after the effective filing date of that application, it was stated that while doping with Zr can result in BZO nanocolumns, it necessitated a “substantially reduced deposition rate”. Selvamanickam et al, “High Critical Current Coated Conductors”, CRADA Final Report For CRADA Number ORNL02-0652 (2011). See also, Foltyn et al, where multiple interlayers of CeO2 are grown within the (RE)BCO film to introduce structural defects, and H. Wang et al. U.S. Pat. No. 7,642,222, issued Jan. 5, 2010, where SrTiO3 layers with varied microstructure are used as the seed layer for growth of the (RE)BCO film.
Other investigators have reported on process dependent limitations, such as critical current saturation as a function of film thickness, for coated conductors formed via pulsed laser deposition. See, e.g., Inoue, “In-Field Current Transport Properties of 600 A-Class GdBa2Cu3O7-δ Coated Conductor Utilizing IBAD Template”, IEEE Transactions on Applied Superconductivity, Vol. 21, No. 3, June 2011. A 2.5 μm thick GdBCO film was made by pulsed laser deposition on a Hastelloy substrate, with a 1.1 μm thick GdZrO7 layer made by ion-beam assisted deposition (IBAD), and a 0.5 μm thick CeO2 layer formed by pulsed laser deposition. A silver protection layer was provided. The measured sample was formed from a 1 cm long piece, formed into a microbridge 70 μm width by 500 μm length. Critical current at 77K and self-field were 600 A/cm-w, and critical temperature was 93 K.
Despite the clear desirability of a coated conductor achieving these objects, the need remains for a comprehensive and effective solution having high yield and low cost in commercially useable lengths.